Fluid flow meters utilizing a floating member have been known. In these known devices, a glass tubular casing constituting a fluid conduit is disposed vertically, and a fluid to be measured is introduced through an inlet pipe provided at its lower end and is discharged through an outlet pipe provided at its upper end. A float is disposed in the casing and is displaceable therein in an amount determined by the volume rate of flow of the fluid passing through the conduit. In these devices, suitable flow rate indicia are provided to enable the flow rate to be visually read or, alternatively or in addition, an optical sensor is provided to optically sense the index or measure the position of the float in the flow meter casing whereby to indicate the fluid flow rate. Inconveniently, these devices are subject to measurement errors when dust obscures the optical path and further requires bothersome maintenance to quite a degree to ensure that the glass flowmeter casing is at all times kept clean; hence they cannot be practically serviced in environments subject to contamination or pollution. Furthermore, the devices of indicia type tend to suffer visual reading errors and themselves cannot be used for the purposes of controlling the fluid flow itself or associated equipments. The devices with an optical sensor require relatively complex structures and hence are expensive to manufacture, and their cost is greater where the aforesaid maintenance problems are taken into account.